Alex Forey, the creator of the Google Calendar Door Sign describes the purpose of his invention:
"It's what I think is generally known as a digital toy: it serves no great purpose other than to be interesting and kinda cool. It's mostly so when someone walks past my door they can see what I'm up to (and why I'm not there)."
What other digital toys have you seen that have caught your eye recently?

Also worth noting: I believe this product could have many use cases, and readers of Hacker News agreed. One of my favorite responses to this product:
"There might be a market for an e-paper based version of this that shows an entire day or week of schedule."